Conventional diffraction gratings operate by periodically modulating the phase or amplitude of light propagating through them, potentially splitting incident light into multiple diffraction orders.
Polarization gratings, which periodically modulate the polarization state of light traveling through them, have been known since the 1970s, when initial publications about the more general case of polarization holograms appeared in Soviet journals.
It was soon recognized that the most compelling advantage of polarization gratings over conventional diffraction gratings was the possibility to control the polarization state of the diffracted orders while at the same time making the efficiency in each order dependent on the polarization of the incident light. Initial success at reducing the theory of polarization gratings to practice came in photochromic silver-chloride (AgCl) glass using holography. In this approach, two nearly orthogonally polarized coherent laser beams were superimposed with nearly parallel propagation, creating a standing optical wave with a periodic modulation of the polarization state while maintaining a constant intensity. Since linearly polarized light induced considerable optical anisotropy (linear birefringence) in the materials through the Weigert-effect, the periodic patterns, where polarization is changing from linear to circular and back, were captured as a polarization grating.
This holographic method eventually made a substantial advance when organic materials containing azobenzene moieties were shown to be able to record these polarization holograms as a relatively strong birefringence. In these materials, the azobenzene groups undergo a reversible trans=>cis=>trans isomerization process and an associated orientational redistribution of the chromophores. Research has shown that a variety of azo-containing polymers and also dispersions may also be used.
In many of these polymers, a surface relief grating is also formed during irradiation. Although the primary reason of the surface generation process is not well understood, several theories have tried to explain the existing phenomenology, and it is agreed that the surface relief appears a result of a mass diffusion mechanism. While it can be useful, this surface relief structure diffracts as a phase grating, and does not lead to a modulation of the polarization state of light propagating through it. In fact, this surface relief grating often degrades the unique diffraction properties of polarization gratings since the properties of both are superimposed. Azo-containing materials are colored in the visible so the range of wavelength applicability is limited. In addition, the long-term stability is usually limited, especially when the grating is exposed to light in the absorption band of the material or subject to high temperature thermal treatments common in applications such as LCD manufacturing.
Other materials have also been studied as alternative polarization hologram materials, including bacteriorhodopsin, holographic polymer dispersed liquid crystals, and a porous glass system imbibed with an azobenzene liquid crystal molecule. Lithographic processing of sub-wavelength metal-stripe structures has also been shown to successfully form a polarization grating by inducing a spatially periodic anisotropic absorption. In this approach, a conductive layer on a substrate is patterned into parallel lines with a sub-wavelength pitch (creating a linear polarizer), where the orientation of these lines determines the transmission/absorption axis of the polarizer. This orientation is varied periodically by the lithography at a pitch greater than the wavelength, forming the polarization grating.
This type of grating operates at infrared wavelengths, but the principle also valid at visible wavelengths (but the fabrication is more difficult since the dimensions are substantially smaller). While good optical quality can be achieved, it is an absorbing optical element (typically 50% of incident light is absorbed) and the fabrication process requires substantial photolithographic processing such as is used for semiconductor wafers (clean room environment, expensive shadow masks, photoresist development, wet chemical etching of inorganic conductive layers, etc.).
One recent method for the production of polarization gratings based on liquid crystals is described by Eakins et al, “Zero voltage Freedericksz transition in periodically aligned liquid crystals”, Applied Physics Letters 85, no 10, pp 1671-1673, 2004, using a holographic exposure to photo-polymerize a polarization sensitive alignment layer, and aligning a liquid crystal composition there on.
However, there still remains a need for new high quality polarization gratings that are easy to produce, temperature stable and useful in practical applications.